B'cuda said

I used a piece of 1/2" oak plywood ( pine would be closer to original but splinters easy ) from Lowe's. I wanted mine to last longer than the original and insulate better.
What I did next was my own preference, I sprayed it gloss enamel black, with a couple of coats of gloss polyurethane. I picked up the cleated threaded anchors with screws at Ace hardware to attach it like orignal to the piano hinge. Insulation and carpet on the inside.
I like my music volume to be able to keep up with the exhaust, so resisting the urge to cut and mount speakers in my new tailgate, instead I bought two carpeted 6x9 speaker boxes and set them in the back with enough cord to move them to the trunk, or roof if needed. Man those boxes in front of that oak backstop and surrounded by the back glass sound like a '78 rock concert!
Anyways, 1/2" is what you want, you will have a hard time getting anything thicker in the hinge on the bottom and to latch at the top.

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